In recent years, to share peripheral devices of a computer, share information of document, still pictures, motion pictures, and the like, games, the Internet, and the like, needs for information communication at home are being grown. Consequently, communication networks are in demand not only in offices but also at home.
Recently, the power line communication technique is regarded as a promising technique as a communication technique used at the time of configuring a communication network in a house, and is being developed. The power line communication technique uses the indoor power line as a signal transmission path and is realized by using, for example, an indoor power line as shown in FIG. 12.
As shown in FIG. 12, a power source plug 3A of a transmitter 3 is connected to a connection part (hereinbelow, called outlet) 2A connected at the terminating end of each of a pair of indoor power lines 1A and 1B, and a power source plug 4A of a receiver 4 is connected to the other outlet 4A. The transmitter 3 and the receiver 4 can operate by supplying with power of a commercial power source from the indoor power lines 1A and 1B, and have modems (modulator/demodulator) for power line communication. In the case of performing power line communication, the transmitter 3 modulates a transmission signal to a high-frequency communication signal by using carrier waves of frequency sufficiently higher than the frequency of the commercial power source by a built-in modem, superimposes the communication signal on an AC voltage on the indoor power lines 1A and 1B, and transmits the resultant to the receiver 4.
When the technique is used, for example, only by connecting plural communication devices having communication function (for example, a personal computer, a household electrical appliance with the communication function, and the like) to outlets installed in rooms of a house, communications can be performed among the communication devices. Recently, high-speed communication techniques in a higher frequency band are developed, and commercialization of a home communication network using a high-speed communication technique and a power line communication technique is in demand.
However, the indoor power line is inherently not installed as a communication line, so that the environment is not always adapted to communication. For example, in FIG. 12, when electric devices are not connected to outlets 2C and 2D for connecting electric devices to the indoor power lines 1A and 1B, branch parts B1 and B2 including the outlets 2C and 2D, respectively, become a so-called open stub state, and an adverse influence is exerted on communication quality. That is, the outlets 2C and 2D become open ends (high impedance) where reflection of the high frequency signal occurs, so that a communication signal deteriorates.
Also in a state where a power source plug 5A of an electric device 5 is connected to the outlet 2C, in the case where the input impedance of the electric device 5 is different from the characteristic impedances of the indoor power lines 1A and 1B, reflection of the high frequency signal occurs in the portion of the outlet 2C, and a communication signal deteriorates.
From the above viewpoint, for example, Japanese Patent Laid-open No. 2002-217797 proposes a power line terminating circuit in which an impedance matching circuit is provided between the indoor power lines 1A and 1B and the electric device 5, and an inductor for checking passage of signals and noises is provided between the indoor power lines 1A and 1B and the electric device 5.
A terminating circuit 130 described in Japanese Patent Laid-open No. 2002-217797 includes, as shown in FIG. 13, a resistor 131 and a capacitor 133 connected in series between the pair of indoor power lines 1A and 1B, and an inductor (coil) 134 connected between one end side (side connected to the indoor power lines 1A and 1B) of a resistor 131 and the electric device 5 as a load. The resistance value of the resistor 131 is set substantially equal to the characteristic impedance of the indoor power lines 1A and 1B. A pair of terminals 135A and 135B is socket terminals to which the power source plug 5A of the electric device 5 is plugged in. Via the power source plug 5A, commercial AC power is supplied to the electric device 5. Both ends of the resistor 131 serve as input/output terminals of high frequency signals used for communication. A capacitor 5B is a capacitor of a line filter provided at the input stage of the electric device 5 to be connected to the terminals 135A and 135B or a parasitic line-capacitance of wiring in the electric device 5.
Japanese Patent Laid-open No. 2002-217797 describes that by disposing the resistor 131 having a resistance value equal to the characteristic impedance of the indoor power lines 1A and 1B, the characteristic impedance of the indoor power lines 1A and 1B and the input impedance of the electric device 5 are matched, and attenuation accompanying the reflection of the high frequency signal can be prevented.
Although not directly related to the field of the power line communication technique, for example, Japanese Patent Publication No. Hei 1-24448 discloses a noise filter having narrow pulse width and high energy density and capable of removing noise. The noise filter has, as shown in FIG. 14, a first coil (inductor) 203 and a second coil 204 connected in series between an input terminal 201 and an output terminal 202, a resistor 205 connected in parallel with the second coil 204, and a capacitor 206 connected between the output terminal 202 and the ground. Japanese Patent Publication No. Hei 1-24448 describes that noise of a narrow pulse width is removed by a CR low-pass filter configured by the resistor 205 and the capacitor 206 whereas a power source voltage and a signal including no noise can be passed as they are and supplied to an apparatus (not shown) connected to the output terminal 202.
The technique described in Japanese Patent Laid-open No. 2002-217797 still has, however, a problem which occurs when a general electric device is connected to the terminating circuit 130 for the following reason. As described above, in a general electric device, a line filter using a capacitor is usually included at an input stage. In this case, the branch parts B1 and B2 (FIG. 12) enter a so-called short stub state. Specifically, since series resonance is generated by the capacitor 5B in the line filter on the electric device side and the inductor 134 in the terminating circuit, in the case where the resonance frequency is in the communication frequency band, the input impedance of the terminating circuit becomes lower than the characteristic impedance of the power line. As a result, reflection occurs in the power line terminating circuit, and the communication signal deteriorates.
Also in the case where the capacitor of the line filter is not provided in the input stage of the electric device 5, as shown in FIG. 13, the parasitic line-capacitance of wiring in an electric device to be connected exists, so that a similar problem may occur.
To avoid the influence of the capacitor in the line filter and the parasitic line-capacitance of the wiring in the electric device in such a circuit configuration, there is also a method of setting the inductor 134 to have a very large value. This method, however, has a problem such that the size of the terminating circuit becomes large.
On the other hand, in the noise filter described in Japanese Patent Publication No. Hei 1-24448, the second coil (inductor) 204 is provided in series with the resistor 205. Consequently, when the frequency of a signal increases, the input impedance of the noise filter increases and cannot be maintained to be constant. Therefore, in the case of simply applying the noise filter to the terminating end of the power line, there is the possibility that impedance matching cannot be obtained with respect to the high frequency signal, and reflection occurs.
In any of the conventional techniques, it is difficult to realize a power line terminating circuit adapted to power line communications.